Sec. 74 A 



MOVABLE-APPENDAGE DESIGN 



709 



abaft the propellers. In the event multiple blades 

 are used, the wmg or offset blades should lie 

 abaft the stock axis of the main blade, so that 

 when swung at an angle the wing blades move 

 farther from the centerplane, indicated in dia- 

 gram 3 of Fig. 74. B. Otherwise, as the Aving rudder 

 elements are swung to achieve large angles of 

 attack they move toward the centerline by 

 (1 — cos 8) times their offset distance. In effect, 

 the wing blades back away from the outflow jets 

 they are supposed to utilize. 



All modern ships, merchant or combatant, may 

 be required to dodge aboveAvater and underwater 

 missiles in a future emergency. For a ship requiring 

 a high degree of maneuverability every offset 

 screw propeller whose axis lies within a reasonable 

 distance of the centerline, say not more than 

 one-quarter of the beam, should have a rudder 

 behind it. Every propulsion device on a submarine 

 should have a diving plane in its outflow jet. 



A multiple-skeg stern logically embodies a 

 rudder behind every skeg carrying a screw pro- 

 peller, whether a high degree of maneuverability 

 is specifically called for or not. On the alternative 

 arch stern of the ABC ship, Avith its single large 

 propeller between two skegs, it is logical to hang 

 a rudder on the after end of each skeg. Fig. 74. K 

 of Sec. 74.15 shows how this is done. 



When shifting from a single rudder to twin 

 rudders in a design, without other major changes 

 in the hull, it is good practice to give each twin 

 rudder a blade area of about 0.6 to 0.7 the area 

 of the single rudder. This is justified by the 

 increase in turning effort achieved with the larger 

 total area. It is realized that the total weight of 

 twin rudders, supports, and steering gear, based 

 on only 0.5 the area of a single rudder, is almost 

 certainly greater than for a single rudder. In 

 other words, if the increased weight of double 

 rudders is accepted, it is good design to make it 

 really worth while by increasing the rudder effect 

 at the same time. However, this should not be 

 carried so far that the turns made by the vessel 

 are excessively sharp and the ship speed in the 

 turn is so greatly reduced that its maneuvering 

 characteristics are impaired. 



The minimum transverse distance between the 

 stock axes of the blades of multiple rudders is, if 

 practicable, made equal to or greater than the 

 maximum blade length of each rudder, measured 

 fore and aft, so that the pressure field of one will 

 not interfere too much with that of the other, 

 when_^they are fully angled. 



A shallow-draft pushboat, with limited rudder 

 depth, and with need for steering and maneuvering 

 the entire puish as well as itself, requires multiple 

 rudders to achieve the necessary lateral forces 

 and turning moments. For maneuvering in ex- 

 tremely shallow water, when the bed clearance is 

 measured in inches rather than in feet, and when 

 traveling in swift, turbulent currents, the craft 

 requires multiple rudders for itself alone, to say 

 nothing of the need for maneuvering when other 

 craft are being pushed. 



In many river craft propelled by sternwheels, 

 the rudders are placed ahead of rather than abaft 

 the propulsion devices. Here they do not work 

 in an outflow jet of augmented velocity, so that 

 the equivalent effect is achieved only by fitting 

 multiple rudders of larger total area. The limited 

 draft is also a factor here. Sometimes the multipli- 

 cation of rudder area ahead of the sternwheels on 

 these vessels is not sufficient. To make up for it, 

 one or two "monkey rudders" are hung on a 

 frame abaft the wheels, to take advantage of the 

 increased velocity in the outflow jet. 



74.4 Shaping the Rudder and the Adjacent 

 Portion of the Ship. Fortunately, the designer 

 often has considerable freedom in shaping the 

 stern profile of a ship, and in the contour and 

 position of the rudder(s). Further, he is often 

 permitted to work up alternative stern arrange- 

 ments; in these he can forget tradition and strive 

 for maximum performance. For example, in a 

 single-screw stern departing somewhat from the 

 normal form it may be found possible to work in 

 a sort of flat or shallow-V shelf, well submerged at 

 load draft, over the top of a spade-type rudder. 

 If so, the vessel benefits from: 



(a) An increase in the rudder aspect ratio and 

 the lift coefficient for a limited range of rudder 

 angle. This is due to the close fit and the negligible 

 differential-pressure leakage between the hull and 

 the top of the rudder. To keep the horizontal gap 

 small, at the top of the rudder, bolted palms for 

 connecting the blade to the stock may be placed 

 somewhat below the extreme top of the rudder. 

 The recesses for assembling the bolts may be 

 closed by cover plates. 



(b) Providing the equivalent of a surface plate 

 to prevent undue air leakage from the surface and 

 breakdown of the rudder action when the — Ap's 

 are large 



(c) Providing an excellent internal support for 

 the thrust bearing, tiller, and steering gear. 



